Treatment of ends and edges of burned gypsum boards



Patented Aug. 20, 1929.

UNITED STATES PA T-ENTOFF ICE,

TBEATMEN T 01. ENDS AND EDGES OF BURNED GYPSUM. BOARDS.

No Drawing. Application flied December This invention relates to improvement in the process of manufacturing gypsum boards known to the trade as gypsum plaster wallboard and gypsumplaster board, whereby the full strength of the set and hardened gypsum core sometimes weakened by excess temperature in drying, is restored, and at'the same time efficient paper bond to this gypsum core by reason of this burning in overdrying, is reestablished. I

Gypsum boards are practically universally dried in large multiple deck high tempera ture kilns to secure a large production in comparative small plant space, and in. com- 1 paratively short drying time element. Gypsum board is manufacturedin a continuous sheet form and cut to individual lengths before drying, as required for distribution. These individual lengths of green, hardened board automatically distribute to :the various decks of the drying kiln,'and as the individual boards travel on the drying decks through the high drying temperature of the kiln, care is exercisedto keep the marginal V cut ends of each board in close contact with the marginal ends of the other boards flowing through the kiln on the same dryer deck.

' is care is exercised to prevent the unprotected gypsum core in the cut marginal ends -30. or edges being exposed to the high tem era: ture of the circulating kiln, atmosp ere, which normally is above the calcination-temperature of the vpsum core, and which by excess contact with such marginal edge can partially ealcine it, that is, partially dehydrate the gypsum formed in the green board by the setting of the'calcined gypsum or plaster of Paris usedin the mixture of the core. If the boards in their aligned deck 40' progress through the kiln are not. closely abutted end .to end, but separated by van- 7 able spaces at the marginal edges and ends, .by reason of this hot kiln atmosphere, al-

ways above 215 4 gypsum, the core is'usually partly calcined at its marginal ends or edges from one quarter of an inchto an inch, which calcining is termed in the trade burning, and such V boards are termed burned boards. The marginal edge on a burned board is softer than the body portion of the'board within the marginal; edge, and such burned core has a tendency to crumble out from between the surfacing sheets of paper when the board F.-,' the calcining point of j 30, 1927. Serial No. 243,757.

is in transit and subsequently handled on the job. An additional defect lies in the fact, that the surfacing sheets of paper are loosened from such burned marginal edge or end and easily peeled or torn oil of the inner gypsum core because of the bond between the paper surfacing and the core being destroyed by this burning or partial calcinat iong Ordinarily this partially calcined or burned 'portion of the core at themarginal edges of the board sheets, is more porous than the interior body portion, because in the marginal portion the expelling of some of .the combined water in the set and hardened gypsum has taken place, thus rendering this portion of the core of high rosity and consequently more water absor cut. The board sheets as they come from the kiln are warm, usually above 100 'F'. even after leaving the cooling section of the kiln, and as the boards are removed from the kiln they are piled upon pallets holding perhaps one hundred or more dried gypsum boards. Because the boards are warm as piled on the pallet, the assembled piles of boards slowly cool down to ioomtemperature. We discovered that by using an aqueous fluid to wet the burned marginal edges or ends of the boards when I freshly piled from the kilns and still warm, 4 the margins of the pile of boards, that is, 5 the marginal edges and ends of the individual boards in the pile quickly absorb aqueous fluid almost as fast as it can be :1 plied to such margins. As stated before, t 1e softening of the marginal edge is due to the conversion in the kiln of some of the hydra'ted gypsum to,partially'hydrated gypsum or plaster of Paris, and the. aqueous solution thus absorbed readily hydrates this plaster of 'Paris back to gypsum again, with the result that the marginal ends and edges become hard, and the bond of the paper sur-. facin sheets at such margins is restored, and t us the deleterious efiect of 'over-dry-' ing or burning in the kiln, of these marginal ends and edges is nullified. Moreover, the pallet of boards is losing heat slowly, and continues to throwout sufiicient heat to dry. out the excess aqueous fluid absorbed beyond that necessary to set up hydrate and harden the burned calcined gypsum core, and to restore bond of paper surfacing to the core, and finally the piles. of wetted board become perfectly dry at their marginal edges just as it fresh from the kiln, and the individual board sheets are just as perfect as if the drying process in the kiln had operated in such fashion that no burning or calcining of the core at the marginal edges of the individual boards had taken place.

It is apparent of course, to those skilled in the art of gypsum products, their manipulation and manufacture, that this rehydrating treatment of burned marginal edges and ends can be applied in many ways as most convenient to the particular production process used. It is quite obvious that increased speed in rehydrating can be secured by an aqueous fluid containing substances that accelerate the hydration of plaster of Paris to gypsum, and also it would be recognized that increased hardness can be obtained by using aqueous solutions of other substances which are Well known to impart this characteristic to gypsum products. We desire to incorporate all such aqueous fluid as within our practice because whatever modification of the process is used, the essential feature of accompanying hydration of the burned or calcined portions of the core, is secured. Having thus described our invention, what we claim is:

1. The process of restoring the hardness of burned and softened marginal edges of kiln'dried gypsum board, comprising the assembly of warm boards into piles, wetting the edges and ends with an aqueous .fluid adapted to rehydrate the plaster of Paris therein, and evaporating the excess fluid by the retained heat in the assembled pile of treated board.

2. The process of rehydrating partially calcined marginal edges or ends of kiln dried gypsum board, comprising the wetting of such burned margins with an aqueous fluid adapted to convert. plaster of Paris into gypsum while the retained kiln heat evaporates the uncombined excess fluid required for thorough wetting of said margins.

3. The process of restoring the bond of paper surfacing to a partially calcined or burned gypsum board core layer at its marginal edges, comprising assembling warm boards into piles, wetting the burned margins with an aqueous fluid adapted to satu-' rate the paperand core body at such margins, and to rehydrate the plaster of Paris therein, and evaporating the excess fluid by the retained heat 'in the assembled piles of treated gypsum board.

4. The process of rehydrating burned marginal edges of kiln dried gypsum board to restore the bond of paper surfacing to the core at such burned margins, comprising wetting the burned margins with an aqueous fluid, adapted to saturate the paper and core at such margins, and to rehydrate the plaster -of Paris therein, which in crystallizing to s. L. AVERY, JR. M. H. BASQUIN. 

